A company can serve dozens of customers and still rely on one account for a big part of its sales. That does not tell you on its own whether the business is a good acquisition. It tells you where to ask a few more questions: what the customer contributes, what keeps the relationship in place, and what could change after a sale.

Start with sales by customer for at least the last three years and the current year to date. Make sure the same customer is grouped consistently if it buys through related companies, different locations, or another name. Reconcile the schedule to the company's financial records. A percentage is useful only when you know what was counted.

A public filing shows why context matters. Exzeo Group's 2025 annual report filed with the SEC reports two customers, each above 10% of revenue, that together accounted for about 93.5% of the year’s sales. The filing separately discusses receivables from large customers and sales to customers affiliated with its controlling shareholder. That is one unusually concentrated public company, not a benchmark for a privately owned acquisition target or a percentage rule for buyers.

Check what the customer contributes

Sales are not the same as profit. Compare the customer's share of revenue with the gross profit or contribution margin the account produces. Ask about discounts, dedicated staff, special inventory, shipping, or service costs. A large account may be less valuable than its sales suggest if it takes more work or leaves less margin.

Then look at payment history and open receivables. A profitable account that pays slowly can tie up cash. That is a different issue from whether the customer may leave, but both affect what a buyer needs to fund. Ask for aging detail and compare it with the customer's normal payment terms.

Find out what holds the relationship together

Read the agreement, purchase orders, renewal terms, pricing provisions, and cancellation rights. Ask transaction counsel to review any assignment or change of control terms. Do not assume a customer contract continues unchanged when ownership changes.

Also ask who manages the relationship day to day. Does the customer work with several people at the company, or does nearly everything run through the owner or one salesperson? Look for evidence in account reviews, service records, renewal history, and regular contact with the customer's team. A relationship can survive a sale more easily when the knowledge and service are shared across the business.

Test a change in the account

Ask management to show what happens if the customer reduces orders or leaves. Separate the sales that disappear from the costs the company can actually remove. Consider how long it could take to replace the work, whether another customer would need different capacity, and what happens to cash during the gap. Use the company's own history and documented pipeline to support the assumptions.

There is no universal percentage that makes an account too large. A significant customer may have a stable, long-running relationship and clear terms. A smaller account may still create risk if it controls key volume or is tied closely to the owner. The useful answer comes from the economics and evidence behind the number.

Questions to answer before setting a value

  • How much sales and gross profit came from the largest customers each year?
  • Are sales grouped consistently across related companies and locations?
  • What do the contract, renewal, cancellation, and ownership-change terms say?
  • Who at the company owns the customer relationship and handles the work?
  • How much receivable balance is open, and how quickly does the customer pay?
  • If orders fell, what costs could be removed and how long might replacement take?

A large customer is a diligence question, not an automatic pass or discount. Trace the sales to profit and cash, understand the contract and people behind the account, and test a realistic decline before deciding how much of the relationship a buyer can count on.